In any social event, including but not limited to a pre-arranged gathering or a random encounter, people usually like to identify themselves as belonging to a certain group or an organization. The members of such a group or organization usually share a common interest or a goal and therefore are willing to identify other members who are also present. However, in many situations, this is not an easy task. For example, at a music concert, the members of a music club or a group of friends would like to know if there are any other members nearby. Unless these group members know one another and agree upon a time and place beforehand to meet at the concert, it is almost impossible to know and detect the presence of the other members or friends. Although these group members can wear an outfit or other personal items that signal their affiliation or affinity, it is still difficult for them to visually detect and recognize one another, particularly in crowded environments. It is therefore desirable to notify a person to the presence of another person sharing the same affinity or interest.